With global water supply shortage problems, RO membrane systems have been currently utilized to desalinate sea water and brine water as well as to recover wastewaters. These RO membrane systems encounter a problematic decrease in performances of RO membranes due to contamination. Thus, demands have been placed on the development of cleaning technique capable of effectively recovering performances of contaminated RO membranes.
Water treating RO membranes used widely in recent years are aromatic polyamide RO membranes which allow for low-pressure operation and exhibit excellent salt rejection performance. However, aromatic polyamide RO membranes have low chlorine resistance and cannot be placed in contact with chlorine under operation conditions unlike cellulose acetate RO membranes. Thus, contamination with microorganisms and organic matters is more likely to occur than in cellulose acetate RO membranes. On the other hand, aromatic polyamide RO membranes have higher alkali resistance than cellulose acetate RO membranes and can be cleaned under alkaline conditions at pH 10 or above.
Conventional cleaning agents which are known to be effective for cleaning such alkali-resistant aromatic polyamide RO membranes of contaminants such as microorganisms and organic matters include:
alkali agents (such as sodium hydroxide),
surfactants (such as sodium laurylsulfate), and
chelating agents (such as EDTA) (Non Patent Document 1).
However, it is sometimes the case that heavily contaminated RO membranes cannot be cleaned sufficiently by these chemical agents.
Sodium hypochlorite is a powerful chemical agent against microorganisms and organic matters. However, the low chlorine resistance of aromatic polyamide RO membranes precludes the use of sodium hypochlorite for the cleaning of aromatic polyamide RO membranes. It is known that when chlorine-based disinfectants are used, they are supplied to RO membranes after free chlorine is reduced with a reducing agent (Patent Document 1).
Patent Document 2 proposes a method of sterilizing and cleaning a membrane module in which the membrane module is cleaned with acid and is thereafter sterilized and cleaned with an oxidizing agent. The types of permeable membranes to be cleaned in Patent Document 2 are not explicitly described but are probably cellulose acetate RO membranes based on the fact that the initial rejection (salt rejection rate) of the membrane is 95%. Patent Document 2 does not disclose that the method is applied to RO membranes with decreased permeation flux or is performed under alkaline conditions.